def test(a,lst=[1,2,3]):
    if a not in lst:
        lst.append(a)
    return lst


print(test(a=5))
print(test(a=15))
# [1, 2, 3, 5]
# [1, 2, 3, 5, 15]
print(test(30, lst=[40, 50]))
print(test(20))